The invention concerns a method of preparing multimetal catalysts.
Multimetal catalysts are frequently used in refining, petrol chemistry, fine chemistry and catalytic combustion and post-combustion, in order to accelerate chemical reactions. They normally have the advantage over monometallic catalysts of being more selective; this is illustrated in Hydrocarbon Technology, 1990, 12, 20-43, by the improvement in selectivity in the formation of toluene during catalytic reforming of n-heptane on a catalyst based on platinum and iridium, compared with the same reaction on a catalyst based on platinum only. In catalytic combustion or post-combustion, catalysts based on platinum and palladium or based on platinum and rhodium are preferred to catalysts based on platinum only, since they broaden the range of operating conditions under which the catalysts can be used.
For metal catalysts to have good catalytic activity, the metal particles must be in a highly divided state. The principle which is most widely applied to obtain this state is deposition of the metals on a porous carrier. Porous carriers have a large specific surface area, on which the metals can be deposited in the form of very small particles. Multimetal catalysts are obtained on the same principle.
The porous carriers used are selected according to their texture, resistance to high-temperature fritting and chemical properties. Those most commonly used are aluminas, silica, silica-aluminas, zeolites, oxides of titanium, cerium, chromium or zirconium, mixed oxides of aluminium and magnesium or aluminium and nickel, carbides, nitrides, borides and activated carbons. Porous carriers may be impregnated with metals by several methods which are well known in the art. Ch. Marcilly and J. P. Franck have drawn up a list of such methods (Revue Institut Francais du Petrole, 1984, 3, 337-364). The most frequently used are impregnation by exchange and dry impregnation. The first method exploits the cation or anion exchange properties of porous carriers. It comprises dipping the porous carrier in an aqueous or organic solution of metal precursors with a controlled pH. The ionic metal precursors in solution are exchanged with surface locations of the carrier. The second method comprises preparing an aqueous or organic solution of metal precursors and impregnating the porous carrier with a volume of solution equal to its pore volume.
These techniques can be applied not only to the preparation of monometallic catalysts but also to the preparation of multimetal ones. In the latter case the solutions of metal precursors simultaneously contain the precursors of the various metals with which the carrier is to be impregnated. The technique is then described as the co-impregnation method.